Foucault invented a type of pendulum, since named in his honor, which gives direct evidence of the rotation of the Earth. He also demonstrated the Earth's rotation by means of a gyroscope.
Some of Foucault's most important work was in the study of light. He improved the method of measuring the speed of light devised by the French physicist A. H. L. Fizeau. By means of this method he found that the speed of light is inversely proportional to the index of refraction of the medium through which the light is passing. His other discoveries included a polarizer for light and an improved method of making telescope mirrors. Foucault also studied the eddy currents, sometimes called Foucault currents, produced when a copper disk rotates in a magnetic field.
The Foucault pendulum is a pendulum used to demonstrate the rotation of the Earth. It consists of a heavy weight suspended by a long wire from a firm support. The wire is attached to the support in such a way that the pendulum can swing in any direction. Once the pendulum is set in motion back and forth in a certain direction, it tends always to swing in that direction. However, because the Earth is rotating at the same time, the direction of the pendulum's motion changes with respect to the Earth. At the North or South Pole, for example, the direction of the pendulum seems to turn through 360° in 24 hours. Actually, the pendulum has not changed its direction of swing, but the Earth has made a complete rotation under it.
At places other than the poles the rate of change between the direction of the pendulum and the direction of the earth is less. The pendulum therefore appears to change direction more slowly, and it takes more than 24 hours for it to turn through 360. The greater the distance from the poles, the more slowly the pendulum turns. At the equator it does not at all.
The Foucault pendulum was invented by the French physicist J. B. L. Foucault. He first demonstrated it
publicly in Paris in 1851.
